Follow-Up: Decibel 1.2.3 & Pure Music 1.8
In "Listening" in the July 2011 issue, Art Dudley wrote about his experiences with three digital music players for Macintosh computers: Amarra, Decibel, and Pure Music, each of which ensures that the bits it presents to the DAC are the same bits as those stored in the file, and at the same sample rate (footnote 1). That column was prepared in early April; when new versions of Decibel and Pure Music became available in June, I thought it useful to write a short Follow-Up.
Decibel, a bare-bones, playlist-oriented player now in its 1.2.3 version ($33) was written by the author of the excellent Max file-format converter, and will run only on Intel-powered Macs and OS10.5 or later. Audio files are dragged'n'dropped into its playlist window. Clicking the Preferences button brings up a checklist, two items on which optimize the program for maximum sound quality. "Load Files in Memory" does what it says: when a file is highlighted and Play is pressed, it is loaded into RAM before being played, thus ensuring that there is no hard-disk access during playback. The second, "Obtain Exclusive Access," aka "Hog Mode," bypasses the operating system's CoreAudio mixer so that only Decibel can access the audio device to which the audio data are being streamed. These data are therefore not contaminated by the usual system alerts and sounds.
If Decibel's Exclusive Access Mode is enabled and the DAC supports integer input (footnote 2), Decibel will now send audio to the DAC as integers. Although audio files are stored as integers (whole numbers), a Mac internally converts audio data to 32-bit floating-point data, which allows mathematical processing (such as volume control and equalization). The floating-point data are reconverted to16- or 24-bit integer, as appropriate, before being sent to an audio device. Decibel's Integer Mode sends the unprocessed data straight to the device, and while I can't see why there should be any improvement from Integer Modemathematics is mathematicsthe choice of no processing over any processing is always appealing.
Pure Music, now in its 1.8 version ($129) has many more features than Decibel. I don't have room to describe them all, but they include oversampling, crossover functions, metering, sample-rate conversion, and use of audio plug-ins, all described in the excellent manual. Basically, while PM can now operate in a bare-bones mode (called Less Is More), it is usually used to take control of the owner's iTunes library. Although the iTunes window is open while PM is running so the user can see cover art, etc., PM controls the music playbackdon't try to use iTunes' transport controls!
PM, too, offers Hog mode, though warns that "the audio device selected in Audio MIDI Setup should be set to a different device than the one used by Pure Music." Once Hog Mode has been selected and PM relaunched, PM1.8 offers Integer Mode playback if the DAC in use supports nonmixable native integer streams. However, this will bypass all signal processing in Pure Music (including the volume control, upsampling, and metering). One advantage of Pure Music over Decibel is that PM will work with older, G4 Macs, such as the Mac mini I've been using for computer audio since 2006, though PM's Integer Mode is disabled with these models. (Integer Mode apparently requires a Mac with 64-bit architecture.)
A significant new feature offered by PM1.8 is its ability to play DSD-encoded files. Unfortunately, I was unable to try this feature, as I don't currently have any DSD files. That will have to await a Follow-Up to this Follow-Up.
Before I describe my experience of the two programs, some housekeeping tips (footnote 3): First, fit your Mac with as much RAM as possible; modern Macs will take up to 8GB. Even when apparently doing nothing, modern computers have so many processes running that a bare minimum of 2GB will help avoid the computer having to use "virtual memory" (ie, writing data temporarily to disk). Second, keep your music library on a standalone hard drive, and frequently back up that drive to a second drive. Third, if you use a FireWire audio DAC, connect the standalone music disk via USB 2.0. If you use a USB DAC, connect the hard drive via FireWire.
I auditioned both players using my Intel MacBook running Mac OS10.6.8 and fitted with 4GB of RAM using a FireWire-connected Metric Halo ULN-2 DAC. This decodes files with sample rates up to 96kHz, and was powered by a linear DC supply I'd purchased from Wavelength's Gordon Rankin. As the ULN-2 has an analog-domain volume control, I connected it directly to a pair of Classé CT-M600 monoblock power amplifiers, which in turn drove Vivid B-1 speakers. Files were stored on a USB-connected hard drive.
I began with Decibel loading files into RAM and the program claiming exclusive access to the ULN-2. This produced a sound typified by a broad, deep soundstage, a lack of treble grain, and an overall ease. That Decibel will play FLAC files directly was a benefit, as I could play Linn's superb 24-bit/96kHz recording of Mozart's Requiem, with Charles Mackerras conducting the Scottish Chamber Orchestra, without having to transcode the files to an iTunes-compatible format.
Pure Music will also play FLAC files. Changing to PM 1.8 but using it as Art Dudley had done, without Memory Play or Hog Mode enabled, was a step back with the Mozart: the soundstage shrank, the sense of an acoustic around the solo singers was diminished, and the sound lost much of its sense of ease. Switching PM1.8 to Memory Play restored those qualities, though I wasn't sure that Hog Mode made any further improvement. But certainly, Pure Music set to Memory Play and Hog Mode, and Decibel set to Exclusive Access, each sending audio to the Metric Halo converter, produced sounds that were indistinguishable from each other.
Pure Music reported that it was not possible to send Integer audio data to the ULN-2, so to test that aspect of the programs I used either an Ayre Acoustics DX-5 connected to the computer by an AudioQuest Coffee USB cable, or a Halide USB-S/PDIF Bridge connected to the BNC S/PDIF input of a Logitech Transporter. The Ayre and Halide both operate in asynchronous USB mode, and volume control was provided by an Ayre K-5xeMP preamplifier. The music library was stored on a FireWire-connected laCie hard drive.
Here I ran into problems. Decibel's playback was plagued by clicks with both USB interfaces. Pure Music in Memory Play and Hog Mode worked fine, but if I enabled PM's Integer Mode I got the same clicks. Changing to the MacBook's other USB port didn't improve things. The results of a Google search suggested that the problem was my MacBook. Apparently, not all Macs support Integer Mode via their USB ports, due to a hardware bug for which there is no workaround.
Some Internet commentators have said that using a powered USB hub eliminates the clicking, so I tried this with Decibel. Playback through the USB devices was now click-free, and better overall than with the FireWire-connected ULN-2. However, when I switched to Pure Music, that program had now grayed out the Integer Mode checkbox, meaning that this option was now impossible via the USB hub. This, presumably, was why Decibel was now working properly. On the Decibel website, designer Stephen Booth says that, "So as long as you enable exclusive access and have a DAC that supports it, Decibel will send integers to the output device." With a direct USB connection to the Ayre or Halide, Decibel apparently sent integer data and triggered the Mac's hardware bug. With the USB hub, I assume that Decibel no longer recognized the device as being capable of Integer Mode, and so sent normal data.
Investigating the sonic improvement due to operating the programs in Integer Mode will have to wait until I buy a 2011-vintage Mac.
Which of these programs to buy? Whether you require a full feature set or bare-bones functionality will be up to you. Fortunately, Decibel and Pure Music can be downloaded and used for limited trial periods (48 hours and two weeks, respectively), so you can try both.John Atkinson
Footnote 1: Why this isn't always the case is discussed in my 2008 article on computer audio.
Footnote 2: A partial list of DACs that support integer mode can be found here.
Footnote 3: My thanks to Clark Greene for his helpful correspondence.